When a concrete casting is poured, especially in the case of castings larger than a particular size, several castings are required to be made in succession. In each casting, cracks normally form after setting of the concrete as the concrete cures due to unavoidable shrinkage in the concrete. Because all castings shrink overall, adjoining castings are normally provided with an expansion joint. In order to produce slabs which are durable and functional, some elements which may be used to provide the formwork and the like become embedded in the concrete slab, and other elements are arranged outside the formwork and may be re-used in subsequent concrete castings. For convenience, in this specification, the term casting elements is to be taken to include all elements in or associated with the casting unless the context indicates otherwise.
One of the functions of casting elements is to provide for the placement of other casting elements within the concrete slab and/or expansion joint elements about the periphery of the slab at an elevation above the base of the slab. For example, casting elements known as bar chairs are used for the placement of reinforcing mesh and/or reinforcing bars in spaced relationship above the bottom of the slab so that in the cast concrete slab, such reinforcing is embedded inward by a predetermined spacing from the outer surfaces of the slab. Whilst bar chairs perform this function adequately, there are sometimes circumstances where the fixed height of bar chairs and/or their particular configuration make them unsuitable for spacing casting elements above the base of the slab, even if the other casting elements include reinforcing bars.
Concrete slabs generally, and roadways in particular, are difficult to seal properly against the elements. The simplest way of making expansion joints is to first pour a whole concrete slab and then, as setting has begun, divide it into smaller sections separated by grooves saw-cut on the surface of the slab by means of a diamond-blade saw. Subsequent contraction cracks will then appear at the grooves. The edges of saw-cut expansion joints are prone to crumbling and chipping, so they are not suitable for some applications. Saw-cutting the expansion joint adds to the cost on construction. It is normally important that such joint be sealed against moisture and flooding to avoid erosion of the substrate or foundation by insertion of a sealant into the joint.
Sealants are not usually installed in concrete slabs until such slabs are 28 days old because concrete shrinks while curing, most shrinkage occurring in the first 28 days from casting. This can result in joint opposing faces becoming contaminated and hard to adhere to. Construction timetables can be difficult to maintain, particularly if weather conditions do not leave the job in a dry condition required for continuation.
Shrinkage can occur after placing sealant and for twelve months or longer after the initial casting. Although sealants are designed to expand and contract, there is a mechanical limit to the extent of expansion. For example, 2 mm of shrinkage in a 10 mm wide joint would result in sealant failure. Moreover, if sealants are left in extension for an extended period of time, the sealant may also fail. One approach has been to make contraction cracks wider so that the proportionate expansion of the crack is less. For example, cracks up to 20 mm wide have been tried, but these do not always work. Moreover, such wide contraction cracks and expansion joints may result in a surface unsatisfactory for wheeled vehicles travelling across the expansion joint.
Conventional expansion and construction joints are traditionally made using timber former to form an edge. After pouring, screeding and sometimes further shaping the concrete into its final form, the concrete is allowed to set. The form work is removed some time after setting of the concrete. Once initial shrinkage has taken place the joint former, for example, styrene or other flexible foam joint expansion material, such as Ableflex® brand elastomeric adhesive is glued to the edge. Then the second slab is placed using the concrete upper edge of the previously cast slab as an aid or guide for screeding the next slab. After 28 days the styrene is removed and the joints are sealed with a flexible sealing agent.
The casting element often used for expansion joints may be referred to as an expansion joint element and is typically formed from metal sheeting with a channel running its length. The channel provides a corresponding ridge on the face of the sheeting opposite the channel. Of course, instead of a channel, one or more concavities of other form may be formed into one or both faces, each concavity thus forming a corresponding convexity protruding outward from the opposite face of the sheeting, so long as the concavities and convexities permit the adjoining edges of cast concrete slabs to move towards and away from one another in substantial alignment with their general planarity. In this specification, unless the context indicates otherwise, the term key former will be used to refer to such expansion joint elements.
Expansion joints formed with a key former still have problems associated with sealing. For example, the key former can interfere with the installation of a sealant in a number of ways. An expansion joint caused solely by the shrinkage of concrete may be too narrow for a sealing agent to be installed, with the result that the expansion joint may not be watertight.
In this specification, terms indicating elevational disposition such as upper, lower, top, bottom, horizontal and vertical may be used to refer to features, parts, portions or the like of the casting elements in their normal orientation, and are not to be taken to limit the use of the casting elements of the present invention to any particular orientation, whether or not in use.
The present invention aims to provide one or more types of concrete casting element which alleviate one or more of the problems associated with concrete castings. Other aims and advantages of the invention may become apparent from the following description.